In a copying machine or a printer using an electronic process, it is known that a toner image formed on a photosensitive drum is transferred to a transfer member, and thereafter the melted toner image by a fixing device including a heating roller and a pressurizing roller is fixed to the transfer member.
Furthermore, an induction heating system is known in which, in the above case, the surface of the heating roller is heated using a plurality of coils. In a case where the plurality of coils are utilized, cost might increase as compared with a case where one coil is utilized. In this case, circuits to drive the plurality of coils must be prepared in accordance with the number of the coils, which leads to the cost increase, and in addition, there rises a problem that the whole device is enlarged.
Moreover, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-151470, in a case where a temperature of a conductive member for use in the heating roller exceeds the Curie point, a skin effect deepens, and therefore the conductive member does not generate any heat. This is utilized, and heating of the heating roller is stopped at a time when it is detected that a temperature of the heating roller rises to an abnormal temperature. In this known technology, in a case where the temperature of the whole heating roller exceeds the Curie point, there is not any problem even when power supply is stopped with respect to a coil which supplies a magnetic field to the conductive member of the heating roller. However, in a case where a small-sized sheet continues to be passed, the temperature reaches the Curie point on the only surface of the heating roller in a portion through which any sheet does not pass, and the conductive member of this portion has an increased depth of penetration. Therefore, any heat is not generated from the only heating roller of the portion through which any sheet does not pass. In this case, since the driving circuit for supplying the power to the coil is not matched with the heating roller, it becomes difficult to heat an only area that passes the sheet.